Direct contacts between fibers from the ventrolateral thalamic nucleus and frontal cortical neurons projecting to the striatum: a light-microscopy study in the rat.

The aim of the present study was to improve our knowledge of the synaptic organization of one major motor loop, the thalamo-fronto-striate system. We examined, within the motor and sensorimotor (Fr1-3, FL, HL) cortices, the organization of the synaptic articulation between thalamic terminals and fronto-striatal neurons. An anterograde tracer (Phaseolus vulgaris leuco-agglutinin) was used to label the fibers from the ventrolateral (VL) thalamic nucleus and a retrograde tracer (subunit b of the cholera toxin) was employed to label the neurons of the frontal cortex projecting to the dorso-lateral quadrant of the caudate-putamen. Our findings indicated that anterogradely labeled fibers arising from the VL formed appositions, presumably synaptic contacts, with retrogradely labeled corticostriate neurons within layer V and, to a lesser degree, within layer III of the frontal cortex. These direct contacts were axo-somatic or axo-dendritic. These findings suggest that some aspects of the motor control mediated by the thalamo-fronto-striate loop could be partly achieved by monosynaptic circuits.